Hi guys,
I have a 79 L82 four speed with factory a/c, good radiator, new coolant, newish 180 degree thermostat. The factory electric fan does work but I'm not sure that it works with the a/c not working -it has a gas leak somewhere that I need to fix. My problem is that when running on the highway in air temp of about 55 degrees my temp gauge shows just below 220 most of the time but then climbs right to the top of the red zone, stays there for maybe 10 or 15 seconds and then quickly drops back to normal again. It stays at normal for a short while and then does it again, back to the red zone. I've checked both radiator hoses (new) and the t stat with my I/ r gauge and the engine does not get that hot. Would the cause be the sender unit or the gauge or the resistor on the back of the gauge or maybe wiring? Also is the sender in the rear on the passenger side? There is another sender in the drivers side towards the front but I don't know what that one is for .
Help? Cheers.

Is this the stock temperature gage you are talking about? If the engine is not overheating, gage or sending unit might be bad or not correctly paired to the gage. The green wire is for the factory gage, it is connected to the temperature sending unit on the driver side head. The other sending unit might be for the fans, did you install the fans? Pretty confusing st this point. Pictures of fan wiring might help.

yes it's the stock temperature gauge and the stock electric fan. 1979 L82s came with an electric fan because they ran hotter. I started the car and let it idle in the garage for about 30 minutes and the temp only got up to 160 degrees with my I/r gauge on the thermostat housing which shows on the gauge in the car as just under 220. It didn't go up into the red at all. I've ordered a new temp sensor anyway.
Thanks for your help.

That’s a great idea. I’ll do that as soon as I get the replacement sender unit. I’ll test them both just as a check.
I had wondered about air in the system and also about whether the water pump might not be pumping enough water through the engine. The pump is quiet so I don’t think that’s the problem. Last time I replaced the coolant I jacked the right front of the car up as high as I could to get as much air out as possible but with the radiator cap not being on top of the radiator it seems impossible to get all the air out. Unless there’s another way that I don’t know about.
Thanks

Is the air dam installed?
​​​​​​​check your coolant thermostat the same way in hot water, it should open at 160 or 180 degrees.

Yeah it has the air dam. I checked the t stat and it opens at about 185 degrees. It's a 180 t stat.

OK, got the new sender unit in the mail, hooked to up with a couple of leads to the ground and the wire to the cars gauge, dropped it in the pot and boiled the water.
The gauge quite quickly went right up the top of the red zone.
I'm guessing that means the gauge is faulty because it certainly wasn't 280 degrees in the pot, only 212.
i know Wilcox makes a new resistor for the back of the gauge. Do you think that might be the problem?
Thanks.

Is the temperature gage wire (dark green) being used for anything else? The resistor on the gage discolored or does the solder connection look good. Take it to an electronics store, they should have the resistor in their shop too.

I don't know if the temp gauge wire is used for anything else, it's not connected to anything else in the engine bay, just goes into the harness. I haven't removed the gauge so I don't know about the solder connections etc behind there yet either. I'm probably not going to get time to remove it for about two weeks but I'll let you know what I find when I get it out.
my clock doesn't work either so I'll try to fix that as well when it's out.
Thanks.

I asked for part number EL70A which is for a 79-80 and the plastic bag had that sticker on it, unopened so I'm guessing it is the right unit, but who knows how accurate these things are anymore. It's probably Chinese.doesnt have a manufacturer name on it.
Almost everything on my car is as original and I think the gauge is. I'll check the condition of the back of it when I get it apart in about two weeks.
Thanks.

You Tube

I understand the sending units are resistance dependent. Is this strictly after market products? How did GM do this on the assembly line? Everything on the Assembly line tasks are seconds dependent. Does the same issue exist with oil pressure sending units?

With my gauge reading normal, I.e just below 220 most of the time but often going briefly to 280, does that indicate to you that the rheostat on the back of the gauge is failing? If that is the case I'll order one of your adjustable ones but first I have to remove the gauge to see if it's original or not and I won't get time to do that for a couple of weeks.
Thanks.

Also thanks for hose links, I think I've seen most of them already!

Yeah I'm quite sure you're right. The new sender is miles off matched to my gauge.
Might have to get the adjustable rheostat from Wilcox.